Singlet fission in pentacene dimers Johannes Zirzlmeiera, Dan Lehnherrb, Pedro B. Cotoc, Erin T. Chernickd, Rubén Casillasa, Bettina S. Basela, Michael Thossc, Rik R. Tykwinskid,1, and Dirk M. Guldia,1 aDepartment of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany; bDepartment of Chemistry, University of Alberta, Edmonton, AB, Canada T6G 2G2; cInstitute for Theoretical Physics & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany; and dDepartment of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany Edited by David R. Reichman, Columbia University, New York, NY, and accepted by the Editorial Board March 9, 2015 (received for review November 26, 2014) Singlet fission (SF) has the potential to supersede the traditional excited states plus a certain amount of phonons are derived as solar energy conversion scheme by means of boosting the photon- the main relaxation products. In other words, the dissipated heat to-current conversion efficiencies beyond the 30% Shockley– leads to a decrease in SF yields and rates (15). Queisser limit. Here, we show unambiguous and compelling evi- In terms of exploiting SF for improving device performances, dence for unprecedented intramolecular SF within regioisomeric pen- e.g., hybrid solar cells, it is necessary to efficiently dissociate – tacene dimers in room-temperature solutions, with observed triplet correlated triplet pairs as they are formed, to overcome triplet quantum yields reaching as high as 156 ± 5%. Whereas previous triplet annihilation (5, 16). Rapid injection of electrons into studies have shown that the collision of a photoexcited chromophore fullerenes, perylene diimides, colloidal nanocrystals, semiconductor with a ground-state chromophore can give rise to SF, here we dem- substrates, etc. suggests a viable strategy. If successful, two charge onstrate that the proximity and sufficient coupling through bond or carriers might be produced per absorbed photon and the photo- space in pentacene dimers is enough to induce intramolecular SF currents of the device can reach external quantum efficiencies of where two triplets are generated on one molecule. more than 100% (5, 17, 18). A provocative debate has been ignited about the mechanism of SF at the molecular level (19, 20). Controversy exists around acene oligomers | excited states | singlet fission | multireference the electronic states that are involved in the process, the coupling perturbation theory | time-resolved spectroscopy among them, and the effective nuclear dynamics (14, 21–27). Two contrasting SF mechanisms have been traditionally postulated–– inglet fission (SF) is a spin-allowed process to convert one the direct and the two-step mechanism. These mechanisms differ Ssinglet excited state into two triplet excited states, namely a in the number and nature of the electronic states that are involved correlated triplet pair (1). The ability to effectively implement SF in the SF process. For the direct mechanism, the nonradiative processes in solar cells could allow for more efficient harvesting relaxation of the initially populated bright state proceeds via a of high-energy photons from the solar spectrum and allow for the correlated triplet pair state of singlet character––sometimes design of solar cells to circumvent the Shockley–Queisser perfor- called multiexcitonic (ME) state––which then dissociates into mance limit (2). Indeed, several recent studies have demonstrated two separated triplet excited states (28). For the two-step CHEMISTRY remarkably efficient solar cell devices based on SF (3–6). mechanism, the relaxation of the bright state occurs via an in- One requirement that needs to be met to achieve SF is that the termediate charge transfer (CT) state to the ME state. As in the photoexcited chromophore in its singlet excited state must share direct mechanism, the ME state eventually allows the two triplet its energy with a neighboring ground-state chromophore. As excited states involved to separate and undergo separate spin re- – such, the potential of coupled chromophores to afford two triplet laxation (11, 21 23). Recent works have, however, challenged excited states via SF has been elucidated in, for example, a tet- these traditional viewpoints on the SF mechanism and several new racene dimer with an SF yield of around 3% (3, 7). Additionally, models have been proposed. In particular, it has been suggested past experiments in single-crystal, polycrystalline, and amor- (13, 29, 30) that the initial excitation produces a coherent super- phous solids of pentacene have documented that the efficiency position of the lowest-lying absorbing state and the ME state, of SF relates to the electronic coupling between these two with the latter splitting into two separated triplet states after chromophores (8, 9). Hence, molecular ordering in terms of crystal packing, that is, proximity, distances, orbital overlap, etc., Significance is decisive with respect to gaining full control over and to fine- tuning interchromophoric interactions in the solid state (10, 11). In the present work, we show compelling evidence for the Of equal importance are the thermodynamic requirements, unprecedented intramolecular singlet fission at room temper- namely that the energy of the lowest-lying singlet absorbing state ature and in dilute solutions within a set of three different must match or exceed the energy of two triplet excited states regioisomeric pentacene dimers. Pump–probe experiments, ≥ (S1 2T1) (11). In light of both aspects, hydrocarbons such as which were complemented by theoretical calculations using –– –– acenes tetracene, pentacene, hexacene and their derivatives high-level ab initio multireference perturbation theory meth- are at the forefront of investigations toward a sound under- ods, corroborate triplet quantum yields as high as 156 ± 5%. standing and development of molecular building blocks for To this end, electronic couplings between the two pentacenes SF. In tetracenes, the singlet- and triplet-pair energy levels are = in the dimers, by virtue of through-bond or through-space in- nearly degenerate (S1 2T1), leaving no or little standard en- teractions, are decisive in tuning the rates of singlet fission. thalpy of reaction for SF (12). In solution, the latter is, however, offset by sizable entropy rendering the process rather slow and, Author contributions: M.T., R.R.T., and D.M.G. designed research; J.Z., D.L., P.B.C., E.T.C., thus, inefficient (13). In addition, the low SF yield relates to the R.C., and B.S.B. performed research; J.Z., P.B.C., E.T.C., R.C., B.S.B., and D.M.G. analyzed dimer geometry. Its nature hinders electronic coupling through data; and M.T., R.R.T., and D.M.G. wrote the paper. space, leaving only through-bond coupling effective. The latter The authors declare no conflict of interest. is, however, insufficient to enhance the SF rate (7, 14). In stark This article is a PNAS Direct Submission. D.R.R. is a guest editor invited by the Editorial contrast, the relaxed triplet excited state in pentacenes has sig- Board. nificantly less than half the energy of the singlet excited state. In 1To whom correspondence may be addressed. Email: [email protected] or dirk.guldi@ turn, the thermodynamic SF requirement, that is (S1 ≥ 2T1), is fau.de. fulfilled for pentacenes rendering this process exothermic and This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. unidirectional (13). Finally, from SF in hexacenes two triplet 1073/pnas.1422436112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1422436112 PNAS | April 28, 2015 | vol. 112 | no. 17 | 5325–5330 Downloaded by guest on September 27, 2021 decoherence. On the other hand, recent theoretical works (14, 27) long wavelength absorption maxima in the 660–678-nm range in have proposed a model for SF in dimers, in which the ME state is toluene and 666–691-nm range in benzonitrile were determined, formed from the absorbing state via a superexchange mechanism together with short wavelength fluorescence maxima at 664 nm involving CT states, although the kinetics of the process is one- (m-2), 676 nm (o-2), and 670 nm (p-2) and fluorescence quantum step–like. All of the above suggests that the traditional classifica- yields ranging from 0.5% to 1.5% in toluene (Table 1). tion of SF mechanisms as direct or two-step is presumably too From the corresponding energetic differences we determined simple to describe the complexity of the process. lowest-lying singlet absorbing-state energies of 1.84 ± 0.05 eV for Until recently, most studies regarding SF have been carried the pentacene dimers o-2, m-2, and p-2. At first glance, all three out in the solid state (11). However, in a groundbreaking report, dimers give rise to absorption patterns that resemble those of Friend and coworkers showed that SF could be observed in so- 6,13-bis(triisopropylsilylethynyl)pentacene (TPc; see SI Appendix lution at room temperature for a pentacene derivative (31). Key for structure), including vibrational fine structure in toluene and to this discovery was the formation of an intermediate state via in benzonitrile in the range from 558 to 691 nm, although the the collision of a singlet excited-state pentacene and a second absorptions of the dimers are red-shifted relative to TPc. A closer pentacene that was in the ground state. This study broke for the look at the ortho-dimer o-2 suggests, however, strong electronic first time, to our knowledge, the dogma of molecular order as a couplings by means of through-space interactions in the ground mandate for SF and demonstrated that the order and packing state. This stems from the close spatial arrangement of the face-to- might not be as crucial as believed, leaving SF as an intrinsic face aligned pentacenes (Fig. 1) as seen in the optimized geom- property even in a state of disorder.